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thesis/chapters/conclusion.tex

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+\acresetall
+\chapter{Conclusion}
+
+% - evaluate effect of transient node resets + hardened implementation
+% - create tools for setting up + measuring + analysing experiments
+% - \fitlab 
+% - stable topology
+%   --> comparable
+% - transient node resets are a thing
+%   - if reset: changes ++
+%   - less with hardening
+%    - if no reset: less good --> depends on use-case
+% - convergence time
+%    - restoring state takes time
+%    - restoring wrong state even more --> validation helps
+% - performance: delay, packet loss
+%   - hardened implementation loses fewer packets when used without restore UID
+%   - but both worse without single node reset
+
+The topic of this work has been to evaluate the reset of single node restarts
+and the possibly resulting transient node failures on a \ac{WSN} which uses
+the \ac{RPL}. The evaluation has been performed using \fitlab. For this software has
+been created to automate the experiments using the existing infrastructure
+provided by \fitlab and analyze the data obtained from the experiments. For
+running the hardened implementation in \fitlab, the implementation has been
+ported to a newer version of \emph{Contiki} that has device support for the
+sensor nodes.
+
+Each experiment has been performed in six different phases, where the first two
+(N and R) use the default implementation of \ac{RPL} in \emph{Contiki}, the
+second pair (H and HR) uses the hardened implementation \cite{mueller2017} and
+the third pair (HS and HSR) uses the hardened implementation that also checks
+the validity of the restored routing information.
+
+First, the \ac{DAG} that results from each experiment has been compared and
+checked for variances. It has been discovered that, using \fitlab it is possible
+to reliably create a tree where a certain node (e.g. the node to be
+reset during R, HR and HSR) joins the \ac{DAG} at a certain rank. This means
+that comparable results can be obtained for the effect of single node resets.
+
+Next, the instability of the \ac{DAG} has been evaluated by counting the number
+of changes that occur during each phase. It has been observed that a single reset
+will trigger a large amount of changes in the \ac{DAG}, which will result in a
+higher energy consumption. If such resets occur, both versions of the hardened
+implementations will reduce the  number of changes to the
+\ac{DAG}, but will increase the number of changes if no resets occur, in which
+case the default implementation performs better.
+
+From observing the network convergence times for \ac{RPL} for each
+implementation with and without single node resets, it can be concluded that
+restoring the state takes time, during which arriving packets will be lost. The
+hardened implementation, but without the validity checks enabled, performs best in
+reducing the time packets are dropped while the node recovers from its reset. It
+has been discovered that the validity checks that involve the exchange of
+\ac{DIO} messages take longer than recreating the state without restoring it
+from persistent memory.
+
+The final part of the evaluation looked at the message overhead generated by
+each implementation and for a network with and without single node resets. Here
+it has been observed, that the default implementation creates the lowest
+overhead for a scenario without single node resets, while the hardened
+implementations create a large overhead if no single node reset occurs. For a
+scenario where single node resets occur, both of the hardened implementations
+manage to achieve a smaller message overhead than the default implementation.
+
+In general, it can be concluded that the energy consumption of the hardened
+implementation is significantly higher than for the default implementation if no
+single node resets occur. If single node resets occur, the hardened
+implementation will only have a small advantage over the default implementation
+in terms of energy consumption.